The present invention relates to a liquid metal cooled directional solidification casting process. More particularly, the invention relates to a liquid metal cooled direction solidification process for casting superalloys.
In addition to composition, the crystal grain characteristics of a superalloy can determine superalloy properties. For example, the strength of a superalloy is determined in part by grain size. At high temperatures, deformation processes are diffusion controlled and diffusion along grain boundaries is much higher than within grains. Hence at high temperatures, large-grain size structures can be stronger than fine grain structures. Generally, failure originates at grain boundaries oriented perpendicular to the direction of an applied stress. By casting a superalloy to produce an elongated columnar structure with unidirectional crystals aligned substantially parallel to the long axis of the casting, grain boundaries normal to the primary stress axis can be reduced. Further, by making a single crystal casting of a superalloy, grain boundary failure modes can be almost entirely eliminated.
Directional solidification is a method for producing turbine blades and the like with columnar and single crystal growth structures. Generally, a desired single crystal growth structure is created at the base of a vertically disposed mold defining a part. Then, a single crystal solidification front is propagated through the structure under the influence of a moving thermal gradient.
During directional solidification, crystals of nickel, cobalt or iron-based superalloys are characterized by a "dendritic" morphology. Dendritic refers to a form of crystal growth where forming solid extends into still molten liquid as an array of fine branched needles. Spacing between the needles in the solidification direction is called "primary dendrite arm spacing." A temperature gradient must be impressed in front of an advancing solidification front to avoid nucleation and growth of parasitic dendritic grains. The magnitude of the required gradient is proportional to the speed of solidification. For this reason, the speed of displacement of the solidification front, which can be on the order of a fraction of a centimeter to several centimeters per hour, must be carefully controlled. Liquid metal cooled directional solidification processes have been developed to meet these requirements. In one process, the alloy material being heated is passed first through a heating zone and then into a cooling zone. The heating zone can consist of an induction coil or resistance heater while the cooling zone is constituted by a liquid metal bath. In another process, the liquid metal bath is utilized both for heating and cooling to provide an improved planar solidification front for the casting of complex articles.
Metals typically used for the liquid metal bath include metals with melting points less than 700.degree. C. Metals with melting points less than 700.degree. C. include lithium (186.degree. C.), sodium (98.degree. C.), magnesium (650.degree. C.), aluminum (660.degree. C.), potassium (63.degree. C.), zinc (419.degree. C.), gallium (30.degree. C.), selenium (220.degree. C.), rubidium (39.degree. C.), cadium (320.degree. C.), indium (156.degree. C.), tin (232.degree. C.), antimony (630.degree. C.), tellurium (450.degree. C.), cesium (28.degree. C.), mercury (-39.degree. C.), thallium (300.degree. C.), lead (327.degree. C.) and bismuth (276.degree. C.). Lithium, sodium, potassium and cesium are very flammable and would present safety issues if used as a liquid metal bath. Magnesium, calcium, zinc, rubidium, cadmium, antimony, bismuth and mercury have low vapor pressures. They would evaporate and contaminate the casting alloy and furnace. Selenium, cadmium, tellurium, mercury, thallium and lead are toxic. Gallium and indium are expensive. Aluminum and tin are preferred coolants. Tin is heavier and more expensive than aluminum, and Tin will contaminate a superalloy if it penetrates through the mold. Aluminum will not contaminate since it is a constituent of most superalloys, but the melting point of aluminum is higher than that of tin. Since heat transfer between a casting and coolant is a function of temperature difference, liquid tin is better than liquid aluminum in removing heat from a casting.
There remains a need to identify a coolant for a liquid metal cooling directional solidification process that has the advantages of tin and aluminum with a melting point less than aluminum and a density and cost less than tin.